The present invention relates to a method of and apparatus for forming a thin film which is employed in a semiconductor device. More particularly, the present invention pertains to a method which is suitable for forming highly-pure thin films of semiconductors, insulators and metals. The present invention is also concerned with an apparatus which is effectively employed to carry out this method.
Plasma CVD (Chemical Vapor Deposition) has heretofore been known as a method of forming dense films of strong adhesion and employed to produce thin films of semiconductors, insulators or metals which are used to fabricate semiconductor devices, as described in Semiconductor Plasma Process Technology, edited and written by Takuo Sugano (Sangyo Tosho, Tokyo, July, 1980), pp. 178-193. However, in most plasma CVD processes, a direct current or an RF (Radio Frequency) discharge is utilized to generate a plasma. Therefore, the resulting plasma spreads over the whole interior of the vacuum chamber for plasma CVD. Accordingly, ions and radicals which are generated as a result of dissociation of a thin film forming gas in the plasma come into contact with electrodes to maintain a discharge or the vacuum chamber wall and react with the materials of these parts to generate impurity elements from the electrodes and the vacuum chamber wall. The resulting impurity elements are transported through the plasma and deposited in the produced thin film as a polluting element, thus causing the quality of the produced thin film to be degraded.
There is another method of forming a thin film wherein a solid material such as germanium or silicon is irradiated with a strong laser beam to rapidly melt, for example, the silicon surface, thereby producing a plasma, and a substrate is irradiated with silicon particles generated in the plasma to thereby form a thin film on the substrate. This method is described in Journal of Vacuum Science and Technology, B3(4) (1985), pp. 968-974, D. Lubben S. A. Barnett, K. Suzuki, S. Gorbatkin and J. I. Greene. According to this prior art method, it is possible to rapidly melt only the solid silicon and produce a plasma by irradiation with a strong laser beam, and therefore it is easy to form a highly-pure thin film having a reduced impurity content. This conventional method suffers, however, from the problem that the silicon which is suddenly melted bumps easily, which results in many silicon fine particles flying into the vacuum from the surface in the form of powder. Since no consideration has heretofore been given to prevention of generation of these fine particles, it has been possible to produce only a silicon thin film containing fine particles having a size of 0.5 to 2.5 .mu.m at a density of 10.sup.5 to 10.sup.6 particles/cm.sup.2.
Thus, the above-described prior arts have the following problems. Namely, in the former prior art, no satisfactory consideration is given to the plasma producing method for forming a highly-pure thin film having a reduced impurity content. As a result, part of the material of the electrodes or the vacuum chamber, which are used to generate a plasma, is deposited in the produced thin film as a polluting element. In the case of the latter prior art method, employment of a plasma producing method by means of a laser beam eliminates the need for electrodes, and since the interaction between the vacuum chamber wall and a plasma produced by a laser is reduced, there has been an improvement in producing a thin film of higher purity than in the case of the former prior art method. However, no satisfactory consideration has been given to the method of preventing the generation of fine particles during the production of a thin film. Accordingly, it is impossible to produce a highly-pure thin film containing no fine particles by the latter prior art method.